Auto merge of #10856 - ehuss:future-incompat-docs, r=epage

Small tweaks to the future-incompat docs.

This contains a few changes to the future-incompat docs to try to make it a little clearer how it works, and some formatting and wording changes.

src/doc/src/reference/future-incompat-report.md

@@ -1,24 +1,37 @@
 ### Future incompat report
 
-Cargo checks for  future-incompatible warnings in all dependencies. These are warnings for
+Cargo checks for future-incompatible warnings in all dependencies. These are warnings for
 changes that may become hard errors in the future, causing the dependency to
 stop building in a future version of rustc. If any warnings are found, a small
 notice is displayed indicating that the warnings were found, and provides
 instructions on how to display a full report.
 
+For example, you may see something like this at the end of a build:
+
+```text
+warning: the following packages contain code that will be rejected by a future
+         version of Rust: rental v0.5.5
+note: to see what the problems were, use the option `--future-incompat-report`,
+      or run `cargo report future-incompatibilities --id 1`
+```
+
 A full report can be displayed with the `cargo report future-incompatibilities
 --id ID` command, or by running the build again with
 the `--future-incompat-report` flag. The developer should then update their
 dependencies to a version where the issue is fixed, or work with the
 developers of the dependencies to help resolve the issue.
 
-This feature can be configured through a `[future-incompat-report]`
-section in `.cargo/config`. Currently, the supported options are:
+## Configuration
 
-```
+This feature can be configured through a [`[future-incompat-report]`][config]
+section in `.cargo/config.toml`. Currently, the supported options are:
+
+```toml
 [future-incompat-report]
-frequency = FREQUENCY
+frequency = "always"
 ```
 
-The supported values for `FREQUENCY` are `always` and `never`, which control
+The supported values for the frequency are `"always"` and `"never"`, which control
 whether or not a message is printed out at the end of `cargo build` / `cargo check`.
+
+[config]: config.md#future-incompat-report